Corn is a major crop used as a human food source, an animal feed and as a source of carbohydrate, oil, protein and fiber. It is principally used as an energy source in animal feeds or as a raw material for the recovery of starch, protein feed fractions, fiber, flaking grits, flour and oil. The number of products produced from corn or components extracted from corn are numerous and include, among others, (i) paper sizings, high fructose corn syrup, adhesives, food thickeners, industrial and medical absorbents and ethanol (from starch); (ii) animal feed and feed components (from whole grain, corn silage, corn gluten feed and meal); and (iii) corn oil (from germ).
Virtually all corn produced in the United States, Canada and Europe and much of the corn produced in South America is produced from hybrid seed. The production of corn hybrids requires the development of corn inbred lines that demonstrate good general and specific combining ability to produce agronomically superior hybrids. Among the traits that plant breeders select for in producing hybrids are high yield potential, good stalk strength, resistance to specific diseases, drought tolerance, rapid dry down and grain quality sufficient to allow storage and shipment to market with minimum loss. The development of these inbreds is both labor and capital intensive, requiring many years for development, followed by evaluation in many different environments. The incorporation of additional traits that further enhance grain quality places additional constraints on the plant breeder, dramatically increasing the time for development of quality corn inbreds.
Once inbreds have been developed, they may be used in several ways to produce F1 hybrid seed. The majority of F1 hybrid seed produced in the United States is of the single cross type. Two inbred lines are intermated to give rise to what is termed an F1 single cross hybrid (A.times.B). In some instances, the female parent in the cross is itself an F1 hybrid, so that a three-way cross hybrid is produced with the genotype of (A.times.B).times.C. More rarely, a four-way cross hybrid is produced, with both male and female parents as F1 hybrids, resulting in a genotype of (A.times.B).times.(C.times.D). In all cases, the resulting kernels from this intermating are sold as seed to growers who ultimately harvest F2 grain from the crop for on farm use or sale. A general review of these systems is available in several texts (e.g., Poehlman, J. M. Breeding Field Crops, 3rd Edition; Avi Publishing Company: Westport, Conn., 1987).
In addition to possessing the proper combination of genetic traits to produce hybrids, the inbreds themselves must be reasonably vigorous to support the demands of modern seed production. This can be illustrated by a description of how single cross hybrids are produced. To control the direction of pollination and assure the harvest of hybrid F1 seed, seed production fields are typically designed so that four rows of inbred corn plants serving as females alternate with one row of inbred corn plants serving as males or pollinators, although other planting patterns that permit the harvest of F1 hybrid seed that is not mixed with the pollinator are possible. The female plants are rendered male sterile either by mechanical detasseling or by biological mechanisms such as cytoplasmic male sterility which renders the tassel nonfunctional. Ovules borne on these female plants are then fertilized by pollen produced by the pollinators that were sown in separate rows to permit the harvest of the F1 hybrid seed. The resulting hybrid seed borne on the female plants (i.e., the seed parents or the seed parent lines) is harvested, cleaned, sized and treated prior to sale to growers. To produce hybrid seed, the pollinator plants need to shed sufficient pollen to fertilize the female plants over a variety of climactic conditions. The hybrid seed borne on the female inbred plants need to be of high quality to allow good germination and early plant vigor in the grower's field and the female plants themselves need to stand and retain cars until the time of harvest. These requirements of the inbred lines themselves further increase the time and money required to develop successful hybrids.
Thus, the capital- and time-intensive development and testing of inbreds are paramount to modern corn production. There are three breeding schemes commonly used to produce inbred lines of corn: the pedigree system of breeding, backcross conversion and recurrent selection. In a commonly practiced form of the pedigree method, two inbred lines of corn, often with different sets of desirable characteristics, are intermated and superior plants are selected and selfed in succeeding generations to become increasingly inbred. Part of this selection procedure involves a periodic assessment of the performance of the emerging inbred lines in various hybrid combinations. The process of continued selfing and selection, typically over five to eight generations, results in the production of lines which are, to a significant degree, genetically homogeneous or inbred. Development and production of an inbred by this method typically takes from 5 to 7 years.
A second method of breeding is backcross conversion, wherein a desired characteristic (generally, one which is simply inherited, such as disease resistance) is introduced into a target inbred (the recurrent parent) by intermating the recurrent parent with a source plant expressing a particular trait of interest. This source plant may also be an inbred, but in the broadest sense can be a member of any plant variety or population cross-fertile with the recurrent parent. The progeny of this cross are then backcrossed (and sometimes selfed) to the recurrent parent, desirable progeny identified and the cycle is repeated. After five to eight cycles of backcrossing and selection, these procedures result in the recovery of the desired characteristic in what is substantially the genetic background of the recurrent parent. Oftentimes the "converted" inbred can be recovered and produced quickly (three to five years) but since the end product is essentially an "older" line in many respects, backcross conversion is generally considered to be a conservative method of inbred development.
The third method of inbred development, recurrent selection, generally involves the extraction of a new inbred from a broad, genetically heterogeneous breeding pool, commonly termed a population. Individual plants within the population are selected for traits of interest such as stalk strength or combining ability and intermated to create a new population from which again to select and intermate individuals with these desired characteristics. Because the number of possible genetic combinations within these populations is quite large, substantial opportunity exists for recovering subpopulations and eventually inbreds with novel grain, seed or whole plant characteristics. However, an inevitable consequence of this genetic diversity is that it takes substantially longer to develop inbreds by recurrent selection than by the preceding two methods.
In summary, all three of the currently available strategies are labor and capital intensive. Each requires many years of effort to allow for both recombination of genetic information and selection to produce inbred lines which would combine to yield hybrid seed which would be sown to produce grain. The rapidity with which satisfactory inbred lines can be developed is determined to a large degree by the nature and number of traits that the lines must possess. The addition of novel or unusual traits, especially if they are controlled by several genes, would significantly increase the time and effort required to produce the desired lines.
Most corn grain is sold and distributed as a commodity, since many of the industrial and animal feed requirements for corn can be met by common varieties of field corn which are widely grown and produced in volume. However, there exists at present a growing market for corn with special end-use properties which are not met by corn grain of standard composition. Most commonly, such "specialty" corn is differentiated from "normal" field corn by altered endosperm properties, such as an overall change in the degree of starch branching (waxy corn, amylose extender; Glover, D. V. and Mertz, E. T. In Nutritional Quality of Cereal Grains. Genetic and Agronomic Improvement; Olson, R. A. and Frey, K. J., Eds.; American Society of Agronomy: Madison, 1987; pp. 183-336), increased accumulation of sugars or water-soluble polysaccharides (sugary, shrunken, supersweet corn; Glover, D. V. and Mertz, E. T., supra) or alterations in the degree of endosperm hardness (food grade corn, popcorn; Glover, D. V. and Mertz, E. T., supra; Rooney, L. W. and Serna-Saldivar, S. O. In Corn: Chemistry and Technology; Watson, S. A. and Ramstead, P. E., Eds.; American Association of Cereal Chemists, Inc.: St. Paul, 1987; pp. 399-429). Specialty corn types are typically grown under contract for production for specific end users who place value on starch quality or other specific kernel quality attributes. An example of this differentiation is the contract production of waxy maize, whereby inclusion of a single homozygous recessive gene (wx) converts normal maize starch (75-80% amylopectin, 20-25% amylose) nearly completely to amylopectin (&gt;99%). In a similar fashion, the recessive gene amylose extender (ae) when homozygous or the dominant gene Ae-5180 when homozygous or heterozygous (Plant Biotechnology, February 1991, Office of Biotechnology, Iowa State University, Ames, Iowa) increases the specific amylose concentration of the corn grain to 50% or greater. Additionally, U.S. Pat. No. 4,798,735 teaches how modified corn starches produced by combinations of simple recessive genes can result in the production of starch with functional properties optimally suited for use in the food industry. Sweet corn is yet another example of a specialty corn product often grown under contract, where the inclusion of the recessive genes sugary, shrunken-2 or sugary enhancer, singly or in combination, confers sweetness through a reduction in the amount of starch and an increase in the amount of glucose, sucrose and/or water soluble polysaccharides normally found in the immature corn kernel (Creech, R. and Alexander, D. E. In Maize Breeding and Genetics; D. B. Walden, Ed.; John Wiley and Sons: New York, 1978; pp. 249-264).
More recently, there is a trend to differentiate corn not only on the basis of alterations in carbohydrate quality but also on the basis of its protein, oil and kernel hardness characteristics. Protein and oil concentration are particularly important determinants of the performance of corn as a component of animal feed (Glover, D. V. and Mertz, E. T., supra; Han, Y., et al. (1987) Poultry Science 66:103-111). Furthermore, as coproducts of wet and dry milling, corn oil and protein are important sources of revenue to wet and dry millers. Recent Iowa State University corn performance trials provide a means for recognizing the industrial value of these corn constituents by reporting not only the yield of tested hybrids but also their calculated wet milling and feed values (Iowa Corn Growers Association, 1989, Higher Processing Value in 1989 State Fair Open Class Corn and Soybeans. Bulletin, Aug. 27, 1989).
The breeding, development and nutritional attributes of high oil corn are described below as illustrative of the state of development, heritability, breeding difficulty and economic advantage attendant to the development of many if not all enhanced quality grain traits. Perhaps the most thoroughly studied high oil corn populations are the Illinois High Oil (IHO) and Alexander High Oil (Alexho) populations developed at the University of Illinois. IHO was developed by modified mass selection within the open pollinated corn variety, Burr's White, over more than 80 cycles of selection commencing in 1896 (Alexander, D. E. In Proceedings of the 43rd Annual Corn and Sorghum Industrial Research Conference, 1988; pp. 97-105; Dudley, J. W. et al. In Seventy Generations of Selection for Oil and Protein in Maize, Dudley, J. W., Ed. Crop Science Society of America: Madison, 1974; pp. 181-212). The highest average kernel or grain oil concentration achieved in this population is about 22% oil on a dry weight basis. In contrast, Dr. Denton Alexander, employing both mass and single kernel selection within a synthetic population (Alexho), was able to achieve an average oil concentration of approximately 22% following 28 cycles of selection (Alexander, supra). A number of corn inbreds have been released from the IHO (R802A) and Alexho (R805, R806) populations and are available to the public through the Director of Agricultural Experiment Station, University of Illinois, Urbana, Ill. None of the IHO or Alexho high oil corn populations have resulted in commercially competitive hybrid varieties.
Oil concentration in corn is a grain quality attribute that is quantitatively inherited (Silvela, L. et al. (1989) Theorelical and Applied Genetics 78:298-304). Several studies indicate that oil concentration of bulked F2 kernels arising from crosses between various Alexho derivatives and inbred lines of normal oil concentration approaches the midparent value of oil concentration of kernels arising from the self-pollination of each parent separately (Alexander, supra; Misevic, D., A. et al. (1989) Crop Sci. 29:613-617). Additionally, F2 grain arising from crossing high-oil and low-oil varieties has been observed to segregate for oil concentration on an individual kernel basis (Alexander, supra). Both of these characteristics are consistent with the hypothesis that oil concentration in corn seed or grain is controlled by the action of several genes, each of which makes a partial contribution to the overall oil concentration. The manipulation of these multiple oil genes makes plant breeding of inbreds for the production of high-oil hybrid seed particularly challenging and time consuming.
Because the genetic heterogeneity is kept high during the initial phases of most recurrent selection programs, it takes substantially longer to develop an agronomically useful inbred from a recurrent selection program than from a program based on pedigree breeding. To date, the majority of high oil corn exists as populations exhibiting varying degrees of genetic nonuniformity. Despite efforts over the last thirty years to develop high oil corn varieties by a combination of recurrent selection and pedigree breeding methods, only a small number of successful high oil inbreds have been produced and only a limited number of high oil hybrid varieties have been grown on any scale.
The widespread demand for high oil corn to meet the needs of poultry, swine, dairy and beef producers is increasingly being met by acceptance of the TopCross.RTM. grain production system. The TopCross.RTM. system is a novel method for the production of corn grain containing enhanced quality grain traits. The method results in the production of grain with enhanced quality traits following the pollination of high yielding plants by plants containing genes for enhanced quality grain traits. High oil corn grain produced by TopCross.RTM. now represents the largest acreage of corn currently grown in the United States other than what is commonly referred to as commodity corn grain. In other words, the acreage devoted to the TopCross.RTM. system exceeds that of white corn, waxy corn, high amylose corn or sweet corn. The pollinator plants with enhanced quality grain traits need not be genetically homozygous (inbred) or even homogeneous in appearance and need not be selected for combining ability with high yielding female plants. In this way, the breeding timeline for the production of successful enhanced quality grain trait pollinators is significantly and dramatically reduced and the production of grain with enhanced quality traits is greatly accelerated. This method has catalyzed a great expansion in the number of available agronomically useful female plants that are being used for the production of grain incorporating enhanced quality traits, thus increasing both the yield and the production range of corn varieties expressing enhanced quality grain traits.
The current TC Blend.RTM. pollinators used in the TopCross.RTM. system may be described as either synthetics or F1 hybrids of synthetics (i.e., synthetic hybrids). They contribute high-oil genes to the developing kernel of the TC Blend.RTM. grain parent plant, causing it to produce a substantially larger embryo or germ than it would otherwise had it been pollinated by plants other than TC Blend.RTM. pollinators. These TC Blend.RTM. pollinators are also sufficiently heterogeneous in the timing of flowering that obtaining synchrony of flowering between the TC Blends pollinator and the TC Blend.RTM. grain parent under field conditions is very often well predicted from small research trials.
However, there are limitations to many of these current TC Blend.RTM. pollinators. Some individual TC Blend.RTM. pollinators demonstrate less potential for vegetative development than the TC Blend.RTM. grain parent with which they are paired. As a result, the TC Blend.RTM. pollinator plants will be unable to secure sufficient light, soil moisture and mineral nutrients due to its being surrounded by larger and more vigorous TC Blend.RTM. grain parents. This will result in the TC Blend.RTM. pollinator plants not achieving their genetic potential. This lack of competitive ability can be exacerbated under conditions of environmental stress. For example, under drought stress, some current TC Blend.RTM. pollinators may be less able to extract soil moisture and/or to survive on reduced soil moisture than the TC Blend.RTM. grain parent with which it is blended. As a result, the ability of the TC Blend.RTM. pollinator to shed sufficient pollen may be adversely affected and/or there may be a significant change in the time when it would shed pollen. Furthermore, the ability of the seed of some current TC Blend.RTM. pollinators to maintain acceptable vigor over time in storage is less than desirable due to the fact that the plant on which the seed is grown is generally weak and prone to disease. As a result, the yield of high quality seed of these TC Blend.RTM. pollinators may be limited. Thus, there exists a need for a class of TC Blend.RTM. pollinators that demonstrate considerably more vegetative vigor and environmental durability and produce higher quality seed than what is generally available.
Another limitation of many existing TC Blend.RTM. pollinators used in the TopCross.RTM. system is their restriction to a narrow geographic range. The current types of TC Blend.RTM. pollinators, both the synthetics and synthetic hybrids, cannot currently be paired with all possible TC Blend.RTM. grain parents. Silk extrusion by some TC Blend.RTM. grain parents may be either too early or too late to be synchronous with pollen shed by these TC Blend.RTM. pollinators.